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The occurrence of shallow and deep-water sedimentary facies has established the Yangtze Platform in South China as a key site for the study of Neoproterozoic ocean oxidation and Ediacaran animal evolution following the Marinoan glaciation. The Yanwutan section in Hunan Province is one of the few coherent sections on the Yangtze Platform where Ediacaran deep-water carbonate sediments (predominantly dolostones) are preserved together with organic carbon-rich shales. Here we present new major and trace element abundance data as well as Sr-, O-and C-isotope compositions of leachates from carbonates of the Doushantuo Formation. We evaluate the role of diagenetic modification of the carbonate rocks and constrain the redox evolution of Ediacaran seawater in space and time. 87 O of cap dolostones at many other sections were compromised by hydrothermal alteration. The overlying organic carbon poor micritic dolostone (unit II) shows negative Ce-anomalies that disappear towards the top of the unit. No Ce-anomalies occur in subsequent organic carbon-rich muddy dolostone units (units III to IV). These observations, enrichments in TOC that correlate with variations in redox-sensitive metals in the carbonates, negative d 13 C carb in units II to IV and the decoupling of d 13 C carb from d 13 C org argue for the existence of mostly anoxic deep-water at the Yangtze passive continental margin during the Ediacaran. The negative Ce-anomalies at the base of unit II (with negative d 13 C carb ) may reflect fluctuations towards suboxic or oxic conditions or an allochthonous origin of this unit. However, trace metal enrichments in carbonates of the same unit argue for reducing conditions in pore-water, whereas the carbonates may have preserved the REE+Y signatures inherited from suboxic-to oxic seawater. The trace element and negative d 13 C carb values in units II to IV are consistent with a stratified basin model with a large partially remineralised organic matter reservoir in anoxic bottom and pore-waters.

Reconstructing the environmental conditions that supported early life on Earth relies on 32 well-preserved geochemical archives in the rock record. However, many geochemical tracers 33 either lack specificity or they are affected by post-depositional alteration. We present a data 34 set of major and trace element abundances and Cd isotope compositions of dome-shaped 35 and conophyton-type stromatolites of the Late Mesoproterozoic Paranoá Group (Brazil), 36 showing distinct values with unprecedented resolution at the lamina scale. 37 The studied stromatolites consist of dolomite with a high purity and a negligible content of 38 immobile elements (e.g., < 0.66 ppm Zr), indicating that elemental compositions are not 39 influenced by detrital contamination. Even though the carbonates have experienced different 40 extent of recrystallization, the measured elemental and isotopic compositions do not 41 correlate with fluid mobile elements. The stromatolites thus represent prime archives for 42 geochemical proxies to reconstruct paleo-environmental conditions. 43 Two endmember compositions can be distinguished by multiple proxy analysis, reflecting 44 the contrasting depositional environments of the two types of stromatolites: Shale-normalized 45 rare earth elements including yttrium (REY SN) patterns of domal stromatolites show a light 46 REY SN (LREY) enrichment (Yb SN /Pr SN < 0.84), slightly super-chondritic Y/Ho ratios (33.6-47 39.3) and unfractionated Cd isotopes relative to upper continental crust. This indicates that 48 the stromatolites formed in a shallow-water environment that was episodically influenced by 49 seawater. Their REY and Cd compositions are dominated by dissolved elements that were 50 delivered via weathering and erosion processes from the ambient continent. 51 In contrast, REY SN patterns of the conophyta are parallel those of modern seawater with 52 an LREY SN depletion relative to HREY SN (Yb SN /Pr SN = 2.1 to 3.9), positive Gd SN anomalies 53 (1.1 to 1.4) and strong super-chondritic Y/Ho ratios (37.9 to 46.2), suggesting a microbial 54 habitat that was dominated by seawater. Cd isotopes correlate negatively with Cd and U, but 55 positively with Mn and Ce concentrations, reflecting authigenic carbonate formation at 56 different depths within a redox gradient of the ancient microbial mat. ε 112/110 Cd dol values 57 increase from-3.52 at the mat surface to +3.46 in the interior of the mat, due to the effect of 58 kinetic fractionation during Cd-uptake, e.g. by adsorption onto organic matter or by 59 precipitation of sulfides, in addition to incorporation into carbonate minerals. Hence, our 60 multi-proxy approach including Cd isotopes bears a high potential to shed light on 61 environmental conditions in ancient microbial habitats and the activity of microbial life on 62 Early Earth.

This is a repository copy of Seawater carbon and strontium isotope variations through the late Ediacaran to late Cambrian in the Tarim Basin.